Digital double sampling method, a related CMOS image sensor, and a digital camera comprising the CMOS image sensor

ABSTRACT

A digital double sampling method, a related complementary metal oxide semiconductor (CMOS) image sensor, and a digital camera comprising the CMOS image sensor are disclosed. The method includes generating first digital data corresponding to an initial voltage level apparent in a pixel in response to a reset signal, inverting the first digital data, outputting a detection voltage corresponding to image data received from outside of the CMOS image sensor, and counting in synchronization with a clock signal, starting from an initial value equal to the inverted first digital data, and for an amount of time responsive to a voltage level of the detection voltage.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Korean Patent Application No.10-2006-111798, filed on Nov. 13, 2006, the subject matter of which ishereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the invention relate to a digital double sampling method,a related complementary metal oxide semiconductor (CMOS) image sensor,and a digital camera comprising the CMOS image sensor. In particular,embodiments of the invention relate to a digital double sampling methodcomprising inverting digital data, a related CMOS image sensor, and adigital camera comprising the CMOS image sensor.

2. Description of the Related Art

CMOS image sensors, which are widely used in digital cameras, convertoptical signals into electrical signals. An operation for converting anoptical signal into an electrical signal is performed by a pixel of theCMOS image sensor, wherein the pixel includes a photodiode and aread-out circuit. The photodiode generates electric charges usingabsorbed light, converts the generated electric charges into an analogcurrent, and delivers the analog current to the read-out circuit. Theread-out circuit converts the analog signal into a digital signal andoutputs the digital signal.

In an analog-to-digital conversion process, a comparator receives ananalog voltage and compares the analog voltage with a ramp voltage. Thecomparator compares the analog voltage with the ramp voltage, and uses acounter to count until the ramp voltage is greater than an analogvoltage. Once the counter stops counting, a count value is digital datacorresponding to an analog voltage, that is, the count value is thedigital data into which the analog voltage has been converted.

An up-down counter is typically used to perform digital double sampling(DDS). DDS means obtaining a difference (Dsig-Drst) between digital dataDrst obtained by converting a first analog signal output by aninitialized pixel into digital data, and digital data Dsig obtained byconverting a second analog signal received from a pixel that hasreceived an external image signal into digital data, wherein the secondanalog signal corresponds to an external image signal.

Because the up-down counter performs up-count and down-count functions,the up-down counter has a relatively large number of gates. Therefore,the up-down counter occupies a relatively large area. In addition, powerconsumption is limited in a high resolution image sensor.

SUMMARY OF THE INVENTION

Embodiments of the invention provide a CMOS image sensor having areduced number of gates, a digital camera comprising the CMOS imagesensor, and a digital double sampling (DDS) method that consumes lesspower.

In one embodiment, the invention provides a digital double samplingmethod of a complementary metal oxide semiconductor (CMOS) image sensor.The method comprises generating first digital data corresponding to aninitial voltage level apparent in a pixel in response to a reset signal,inverting the first digital data, outputting a detection voltagecorresponding to image data received from outside of the CMOS imagesensor, and counting in synchronization with a clock signal, startingfrom an initial value equal to the inverted first digital data, and foran amount of time responsive to a voltage level of the detectionvoltage.

In another embodiment, the invention provides a complementary metaloxide semiconductor (CMOS) image sensor comprising a controllergenerating a clock signal and first control signals, and a pixeloutputting a pixel signal in response to the first control signalsduring a reset mode and during an image input mode, wherein the pixelsignal output by the pixel during the image input mode corresponds toimage data received from outside of the CMOS image sensor. The CMOSimage sensor further comprises a comparator comparing the pixel signalwith a ramp signal and activating a comparison signal when a voltagelevel of the ramp signal is less than a voltage level of the pixelsignal, a counter counting in response to the clock signal to a firstcount value during the reset mode, inverting the first count valueduring the image input mode, counting in response to the clock signaluntil the comparison signal is activated to obtain a second count value,and outputting the second count value. Additionally, the counting inresponse to the clock signal until the comparison signal is activatedbegins from an initial value equal to the inverted first count value.

In yet another embodiment, the invention provides a digital cameracomprising a processor, and a complementary metal oxide semiconductor(CMOS) sensor connected to the processor through a bus. The CMOS sensorcomprises a controller generating a clock signal and first controlsignals, and a pixel outputting a pixel signal in response to the firstcontrol signals during a reset mode and during an image input mode,wherein the pixel signal output by the pixel during the image input modecorresponds to image data received from outside of the CMOS imagesensor. The CMOS image sensor further comprises a comparator comparingthe pixel signal with a ramp signal and activating a comparison signalwhen a voltage level of the ramp signal is less than a voltage level ofthe pixel signal, and a counter counting in response to the clock signalto a first count value during the reset mode, inverting the first countvalue during the image input mode, counting in response to the clocksignal until the comparison signal is activated to obtain a second countvalue, and outputting the second count value. Additionally, the countingin response to the clock signal until the comparison signal is activatedbegins from an initial value equal to the inverted first count value.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described herein with reference tothe accompanying drawings, in which:

FIG. 1 is a block diagram illustrating a digital camera system comprisesa CMOS image sensor in accordance with an embodiment of the invention;

FIG. 2 is a block diagram illustrating a CMOS image sensor in accordancewith an embodiment of the invention;

FIG. 3 is a circuit diagram illustrating a pixel of a pixel arrayillustrated in FIG. 2;

FIG. 4 is a graph of voltage versus time illustrating an analog signalVa and a ramp signal Vr;

FIG. 5 is a circuit diagram illustrating a counter shown in FIG. 2;

FIG. 6 is a circuit diagram illustrating inner circuitry of aT-flip-flop of the counter illustrated in FIG. 5;

FIG. 7 is a flowchart illustrating a digital double sampling method inaccordance with an embodiment of the invention for a CMOS image sensorin accordance with an embodiment of the invention; and

FIG. 8 is a timing diagram illustrating an exemplary operation of theCMOS image sensor of FIG. 2 in accordance with the method of FIG. 7.

DESCRIPTION OF EMBODIMENTS

Throughout the drawings, like reference symbols indicate like or similarelements.

FIG. 1 is a block diagram illustrating a digital camera systemcomprising a CMOS image sensor. Referring to FIG. 1, the digital camerasystem comprises a CMOS image sensor 100, a processor 200, a memory 300,a display 400, and a bus 500. Under the control of processor 200, CMOSimage sensor 100 captures external image data. In addition, the capturedimage data is stored in memory 300 through bus 500. Also, under thecontrol of processor 200, image data stored in memory 300 is reproducedby display 400. CMOS image sensor 100, in accordance with an embodimentof the invention, will now be described in more detail.

FIG. 2 is a block diagram illustrating a CMOS image sensor in accordancewith an embodiment of the invention. Referring to FIG. 2, CMOS imagesensor 100, which converts external image data into digital data andstores the digital data, comprises a timing controller 10, a pixel array20, an analog-digital converter 30, and a buffer 40. In addition,analog-digital converter 30 comprises a comparator 31, a counter 32, anda ramp voltage generator 33.

Pixel array 20, which receives external image data (i.e., receives imagedata from outside of CMOS image sensor 100), outputs an analog signal Vato analog-digital converter 30 in response to control signals Rx, Tx,and Sel received from timing controller 10. Analog signal Va may also bereferred to herein as a “pixel signal.” Comparator 31 receives analogsignal Va, and receives a ramp signal Vr from ramp voltage generator 33.As used herein, the term “ramp signal” refers to a signal, such as rampsignal Vr, wherein the voltage level of that signal increases ordecreases over time (i.e., in proportion to time). Ramp signal Vr mayalso be referred to herein as ramp voltage Vr. The voltage level of rampsignal Vr may, for example, increase or decrease at a constant rate.Counter 32 receives a clock signal CLK, a control signal RST, and acontrol signal BWI. Control signal RST may be referred to herein asreset signal RST and control signal BWI may be referred to herein asinversion signal BWI. In addition, counter 32 counts in response toclock signal CLK while analog signal Va is compared with ramp signal Vr.Comparator 31 compares analog signal Va with ramp signal Vr, and outputsa comparison signal LATCH to timing controller 10 in response todetecting a defined voltage difference between analog signal Va and rampsignal Vr. When timing controller 10 stops providing clock signal CLK tocounter 32, then counter 32 stops counting. When counter 32 stopscounting in response to clock signal CLK stopping, a count value storedin counter 32 is digital data that corresponds to analog signal Va. Thedigital data that has been converted from analog signal Va is stored inbuffer 40. In addition, timing controller 10 transmits a control signalR_Ad to buffer 40 in order to receive a data signal R_D from buffer 40.

FIG. 3 is a circuit diagram illustrating a pixel 22 of pixel array 20 ofFIG. 2. That is, pixel 22, which is illustrated in FIG. 3, is one of theN×M pixels of pixel array 20 of FIG. 2. Referring to FIGS. 2 and 3,pixel 22 comprises four NMOS transistors NT0-NT3 and a current sourceIb. A first transistor NT0, a second transistor NT1, and a photodiode PDare connected to one another in series. A drain of first transistor NT0is connected to a power source voltage, and an anode of photodiode PD isconnected to a ground voltage VSS. A gate of first transistor NT0 isconnected to a control signal Rx, and a gate of the second transistorNT1 is connected to a control signal Tx.

Third transistor NT2 and fourth transistor NT3 are connected to oneanother in series. A drain of third transistor NT2 is connected to apower source voltage, and a drain of fourth transistor NT3 is connectedto a source of third transistor NT2. A gate of the third transistor NT2is connected to a source of first transistor NT0, which is connected toa detection node FD, and a gate of fourth transistor NT3 is connected toa control signal Sel. Additionally, a source of fourth transistor NT3 isconnected to comparator 31 and a current source Ib.

First transistor NT0 initializes pixel 22 in response to a controlsignal Rx received from timing controller 10. Second transistor NT1provides an analog signal received from photodiode PD to detection nodeFD in response to a control signal Tx received from timing controller10. Third transistor NT2 and current source Ib form a source follower.Additionally, an analog signal Va input via the gate of third transistorNT2 is provided to comparator 31. Also, pixel 22 of FIG. 3 is selectedfrom among a plurality of pixels 22 of pixel array 20 in response tocontrol signal Sel, which is provided from timing controller 10 tofourth transistor NT3.

Timing controller 10 activates control signals Rx and Sel to initializea detection node FD of pixel 22. Pixel 22 outputs an initialized analogsignal Va (i.e., an initialized analog voltage Va) to comparator 31.Initialized analog signal Va may also be referred to as an “analogpixel-reset signal”.

Timing controller 10 activates control signals Tx and Sel so that pixel22 will provide external image data from photodiode PD of pixel 22 todetection node FD of pixel 22, and so that pixel 22 will output tocomparator 31 an analog voltage Va corresponding to the external imagedata.

Analog-digital converter 30 performs digital double sampling (DDS) torelatively accurately convert an analog signal output by a pixel into adigital signal. An up-down counter has typically been used as ananalog-digital converter to perform DDS.

FIG. 4 is a graph of voltage versus time illustrating analog signal Vaand ramp signal Vr. Referring to FIGS. 2 through 4, ramp voltagegenerator 33 generates a ramp signal Vr in response to a control signalRon received from timing controller 10. Ramp signal Vr is a voltagesignal that decreases from a reference voltage level Vref at a constantrate (i.e., at a predetermined rate as time elapses).

Comparator 31 compares analog signal Va with ramp signal Vr insynchronization with a clock signal CLK of timing controller 10. At thispoint, timing controller 10 activates clock signal CLK to operatecounter 32.

Referring to the graph of FIG. 4, which corresponds to an exemplaryoperation of comparator 31, from time t1 to time t5, the voltage levelof analog signal Va is less than or equal to the voltage level of a rampsignal Vr. However, in accordance with an embodiment of the invention,once the voltage level of ramp signal Vr has decreased to a voltagelevel that is less than the voltage level of analog signal Va (i.e.,once analog signal Va is greater than ramp signal Vr, which is aftertime t5 in the exemplary operation corresponding to the graph FIG. 4),comparator 31 activates comparison signal LATCH. When comparison signalLATCH is activated, timing controller 10 stops generating clock signalCLK, and when timing controller 10 stops generating clock signal CLK,comparator 31 stops operating and counter 32 stops operating (i.e.,counter 32 stops counting). In addition, when counter 32 stops countingin response to the activation of comparison signal LATCH, the countvalue stored in counter 32 at that time is digital data corresponding toanalog signal Va.

An up-down counter is typically used to perform DDS. However, an up-downcounter occupies a relatively large area because it includes arelatively large number of gates, and power consumption increases whenthe CMOS image sensor operates with relatively high resolution and at arelatively high speed.

In accordance with an embodiment of the invention, an alternate counterfor a digital converter of a CMOS image sensor is provided. A counter inaccordance with an embodiment of the invention uses a two's complementnumber system to perform the function that an up-down counter typicallyperforms in a CMOS image sensor. Inverted digital data A′ of digitaldata A may be obtained by subtracting digital data A from a data valueMAX_CODE, as shown in Equation 1. Likewise, inverted digital data A′ maybe obtained by inverting digital data A (i.e., inverting each of thebits of digital data A). As used herein, “inverting” data or a valuemeans inverting each of the bits of the data or value. As used herein,“MAX_CODE” means the largest code value for a given number of bits. Forexample, a 4-bit MAX_CODE is “1111₂”. In one example, assuming that a4-bit digital data value A is “0101₂”, then inverted digital data A′ is“1010₂”. That is, if digital data value A is “0101₂” (which is “5” inbase ten), then inverted digital data A′ (which is digital data Ainverted) is “1010₂”, which is “−6” in base ten interpreted under thetwo's complement number system, and is “10” in base ten when interpretedwithout using the two's complement number system.

In accordance with an embodiment of the invention, the differencebetween digital data A and digital data B may be obtained using thetwo's complement number system in accordance with Equation 2 below. Whena counter uses the two's complement number system, a value of (B−A)−1 isfound for the difference between digital data values A and B, as shownon the right hand side of Equation 3. That is, one less than thedifference between digital data A and digital data B is found.Therefore, a counter using the two's complement number system accountsfor the additional “−1” term on the right hand side of Equation 3. Thatis, during a reset operation, the counter sets the state of eachflip-flop to the data value “1” (i.e., “high”), and when a clock signalis provided to the counter, the counter begins its counting with thevalue “0” (i.e., “0” is the first value counted).A′=MAX_CODE−A  Equation 1A′+B=MAX_CODE+(B−A)  Equation 2MAX_CODE+(B−A)=(B−A)−1  Equation 3

A DDS counter in accordance with an embodiment of the invention uses thetwo's complement number system so that a result of subtracting a digitalpixel-reset value (i.e., a digital data value corresponding to an analogpixel-reset signal) from a digital image value (i.e., digital dataobtained by converting an analog signal that corresponds to externalimage data into digital data) may be readily obtained. Typically, anup-down counter performs an up-count operation to obtain a digitalpixel-reset value and performs a down-count operation to obtain adifference between the digital pixel-reset value and the digital imagevalue. Since the functionality of an up-down counter is more complicatedthan that of an up-counter or a down-counter, an up-down counteroccupies a relatively large area. As used herein, an “analog pixel-resetsignal” is an analog signal output by a pixel after the pixel has beeninitialized.

In accordance with an embodiment of the invention, the function that anup-down counter typically performs in a CMOS image sensor is realizedusing a counter that counts in one direction (i.e., using a counter thatincreases its count value or a counter that decreases its count value).The counter that counts in one direction may also be described as acounter that counts monotonically. For example, an up-count operation isperformed to obtain a digital pixel-reset value, and then, to obtain adifference between the digital pixel-reset value and a digital imagevalue, the digital pixel-reset value is inverted and then anotherup-count operation is performed on the inverted digital pixel-resetvalue. That is, a DDS counter in accordance with an embodiment of theinvention performs an inversion operation so that the DSS counter mayhave a relatively small area and relatively low power consumption. Asused herein, the term “inversion operation” means a process of invertingdigital data or a digital value. A DDS counter using the two'scomplement number system, in accordance with an embodiment of theinvention, is illustrated in FIGS. 5 and 6.

FIG. 5 is a circuit diagram illustrating counter 32 of FIG. 2 inaccordance with an embodiment of the invention. Referring to FIGS. 2 and5, counter 32 comprises a T-flip-flop (T-F/F). Counter 32 illustrated inFIG. 5 is a 4-bit counter 32, and 4-bit counter 32 comprises fourT-flip-flops TC0 through TC3.

Each of T-flip-flops TC0 through TC3 comprises a first input terminal T(i.e., a toggle terminal T), a second input terminal CK, a third inputterminal RESET, a fourth input terminal INV, and an output terminal Q.First input terminal T of first T-flip-flop TC0 is connected to a powervoltage, and second input terminal CK of first T-flip-flop TC0 isconnected to a clock signal CLK received from timing controller 10.

Second input terminal CK of second T-flip-flop TC1 is connected tooutput terminal Q of first T-flip-flop TC0. Second input terminal CK ofthird T-flip-flop TC2 is connected to output terminal Q of secondT-flip-flop TC1. Also, second input terminal CK of fourth T-flip-flopTC3 is connected to output terminal Q of third T-flip-flop TC2. Firstthrough fourth T-flip-flops TC0 through TC3 each further comprise thirdinput terminal RESET for initialization and fourth input terminal INVfor inverting data.

FIG. 6 illustrates inner circuitry of first T-flip-flop TC0 of counter32 (i.e., DDS counter 32) illustrated in FIG. 5 in accordance with anembodiment of the invention. The inner circuitry of each of secondthrough fourth T-flip-flops TC1 through TC3 is similar to that of firstT-flip-flop TC0 illustrated in FIG. 6. However, second through fourthT-flip-flops TC1 through TC3 do not receive clock signal CLK, forexample.

Referring to FIGS. 5 and 6, first T-flip-flop TC0 comprises a firstinverter 32_1, a second inverter 32_2, and a third inverter 32_3, eachof which is controlled by control signal BWI and clock signal CLK. FirstT-flip-flop TC0 also comprises a transmission gate 32_5, which iscontrolled by control signal BWI, and an NMOS transistor 32_4, which iscontrolled by control signal RST.

First, second, and third inverters 32_1, 32_2, and 32_3 are connected inseries. In addition, a signal on output terminal Q of first T-flip-flopTC0 is fed back to an input of first inverter 32_1 through transmissiongate 32_5. An output of second inverter 32_2 is connected to a drain ofan NMOS transistor 32_4. NMOS transistor 32_4 is controlled by controlsignal RST, and a drain of NMOS transistor 32_4 is also connected to aninput of third inverter 32_3.

When counter 32 is operating normally, counter 32 initializes an outputvalue of counter 32 in response to an activated control signal RST. Thatis, when reset signal RST is activated, each of first through fourthoutputs Q[0] through Q[3] of first through fourth T-flip-flops TC0through TC3, respectively, is set to the data value “1” (i.e., a logichigh level). Referring to FIG. 6, during an up-count operation ofcounter 32, a value output from first T-flip-flop TC0 is input to thirdinverter 32_3 of first T-flip-flop TC0 via first and second inverters32_1 and 32_2 of first T-flip-flop TC0 in accordance with clock signalCLK. In addition, an output of third inverter 32_3 is delivered to anoutput terminal Q.

During an inversion operation in counter 32, the output value of firstT-flip-flop TC0 is not input to first inverter 32_1 via transmissiongate 32_5, which is deactivated in response to an activated controlsignal BWI. Also, in response to activated control signal BWI, firstT-flip-flop TC0 inverts a data value (i.e., a bit) input to firstinverter 32_1 through first, second, and third inverters 32_1, 32_2, and32_3. In addition, an output of third inverter 32_3 is delivered tooutput terminal Q.

FIG. 7 is a flowchart illustrating a digital double sampling method fora CMOS image sensor in accordance with an embodiment of the invention.FIG. 8 is a timing diagram illustrating an exemplary operation of theCMOS image sensor of FIG. 2 in accordance with the method of FIG. 7.Referring to FIGS. 2 through 8, a digital double sampling operation ofCMOS image sensor 100 is roughly divided into a reset mode and an imageinput mode. The reset mode is divided into a reset operation S1 and areset analog-digital-conversion (ADC) operation S2. The image input modeis divided into an inversion operation S3, a signaling operation S4, anda signal ADC operation S5.

In reset operation S1, in response to control signals Rx and Selreceived from timing controller 10, a pixel 22 of pixel array 20 isinitialized and pixel 22 outputs an initialized analog signal Va (i.e.,an analog pixel-reset signal). Also, in response to reset signal RSTfrom timing controller 10, a count value stored in counter 32 is reset(i.e., initialized).

In reset ADC operation S2, timing controller 10 deactivates controlsignal RST and activates clock signal CLK to convert initialized analogsignal Va (output by pixel 22) into a digital signal Drst. A digitaldata value obtained by converting an initialized analog signal Va into adigital data value is defined as Drst. However, because counter 32begins with 0 when counting during reset ADC operation S2, counter 32actually stores a digital value Drst−1 at the end of reset ADC operationS2. As an example, in the exemplary digital double sampling operationillustrated in FIG. 8, at the end of reset ADC operation S2, whiledigital value Drst has a value of 6, counter 32 actually stores adigital value Drst−1, which has a value of “0101₂” (which is “5” in baseten).

In inversion operation S3, counter 32 inverts digital value Drst−1 inresponse to the activation of control signal BWI of timing controller10. Thus, in the exemplary operation illustrated in FIG. 8, digitalvalue Drst−1, which is “0101₂” (or “5” in base ten), is inverted toobtain a digital value −Drst, which is “1010₂” (or “−6” in base tenunder the two's complement number system).

In signaling operation S4, pixel array 20 receives external image dataand outputs a new analog signal Va (i.e., a detection voltage Va)corresponding to the external image data. Digital data Dsig is obtainedby converting analog signal Va corresponding to the external image datainto digital data.

In signal ADC operation S5, timing controller 10 activates clock signalCLK provided to counter 32, and counter 32 calculates a data valueDsig−Drst in response to clock signal CLK. That is, because counter 32starts counting from −Drst in signal ADC operation S5, at the end ofsignal ADC operation S5, a count value stored in counter 32 will beDsig−Drst. For example, in the exemplary operation illustrated in FIG.8, when counter 32 begins counting, the value stored in counter 32 isdigital value −Drst, which is “1010₂” (or “−6” in base ten under thetwo's complement number system), so, when counter 32 stops counting,counter 32 will store a count value of Dsig−Drst, which, in the exampleillustrated in FIG. 8, is equal to “1000₂” (or “8” in base ten when thevalue is not interpreted under the two's complement number system).

An embodiment of the invention provides a digital double samplingcounter that counts in only one direction but can also perform thefunction typically performed by an up-down counter in a CMOS imagesensor using the two's complement number system. Also, an embodiment ofthe invention provides a digital double sampling counter that has arelatively small number of gates and that consumes a relatively lowamount of power.

In addition, an embodiment of the invention provides a CMOS image sensorhaving a counter that counts in only one direction but can also performthe function typically performed by an up-down counter in a CMOS imagesensor using the two's complement number system. Also, an embodiment ofthe invention provides a digital double sampling method that uses arelatively small number of gates and consumes a relatively low amount ofpower.

Although embodiments of the invention have been described herein, theembodiments may be modified by one of ordinary skill in the art withoutdeparting from the scope of the invention as defined by the accompanyingclaims.

1. A digital double sampling method of a complementary metal oxidesemiconductor (CMOS) image sensor including a pixel, the methodcomprising: using a counter operating in response to a clock signal andbeginning at a reset value, generating a first count value related to afirst signal provided in response to reset of the pixel; then invertingthe first count value within the counter to generate an inverted firstcount value; and then, using the counter and beginning at the invertedfirst count value, generating a second count value related to a secondsignal provided in response to application of external image data to thepixel.
 2. The method of claim 1, wherein the counter is a single countercircuit that counts in one direction from the reset value in response tothe clock signal and generates the first count value, and counts in theone direction from the inverted first count value in response to theclock signal and generates the second count value.
 3. The method ofclaim 1, wherein generating the first count value comprises: generatinga ramp signal; comparing a level of the ramp signal with a level of thefirst signal to generate a comparison signal; and beginning a firstcount by the counter at the reset value and continuing the first countuntil the comparison signal is activated.
 4. The method of claim 3,wherein the level of the ramp signal decreases until the level of theramp signal falls below the level of the first signal to activate thecomparison signal.
 5. The method of claim 3, wherein generating thesecond count value comprises: generating the ramp signal; comparing alevel of the ramp signal with a level of the second signal to generate acomparison signal; beginning a second count in the counter at theinverted first count value and continuing the second count until thecomparison signal is activated; and providing the second count valuewhen the comparison signal is activated to stop the second count.
 6. Themethod of claim 5, wherein the level of the ramp signal decreases untilthe level of the ramp signal falls below the level of the second signalto activate the comparison signal.
 7. The method of claim 5, whereincontinuing the second count comprises increasing or decreasing a valuein the counter.
 8. The method of claim 1, wherein the first signal is ananalog pixel-reset voltage provided when the pixel is reset byapplication of a reset signal.
 9. The method of claim 1, wherein thefirst count value comprises multiple bits, and inverting the first countvalue comprises inverting each and every one of the multiple bits. 10.The method of claim 1, further comprising: storing the second countvalue in a buffer, wherein the second count value is a digital datavalue indicating to a difference between a level of the first signal anda level of the second signal.
 11. A complementary metal oxidesemiconductor (CMOS) image sensor comprising: a controller thatgenerates a clock signal and a reset control signal; a pixel thatoutputs a first pixel signal in response to the reset control signalresetting the pixel, and outputs a second pixel signal in response toexternal image data applied to the pixel; and an analog to digitalconverter comprising: a comparator that compares a first signal relatedto the first pixel signal with a ramp signal, and first activates acomparison signal in response to the comparison between the ramp signaland the first signal, and compares a second signal related to the secondpixel signal to the ramp signal, and again activates the comparisonsignal in response to the comparison between the ramp signal and thesecond signal; and a counter that counts in response to the clock signaluntil the comparison signal is first activated to generate a first countvalue, then inverts the first count value to generate an inverted firstcount value, and then again counts in response to the clock signal untilthe comparison signal is again activated to generate a second countvalue.
 12. The CMOS image sensor of claim 11, wherein the counter countsin response to the clock signal to generate the first count valuebeginning at a reset value for the counter.
 13. The CMOS image sensor ofclaim 12, wherein counting in response to the clock signal by thecounter to generate the second count value comprises increasing ordecreasing a value in the counter beginning at the inverted first countvalue.
 14. The CMOS image sensor of claim 11, further comprising a rampvoltage generator that generates the ramp signal.
 15. The CMOS imagesenor of claim 11, wherein the first count value comprises multiplebits, and inverting of the first count value by the counter comprisesinverting each and every one of the multiple bits.
 16. The CMOS imagesensor of claim 11, wherein the counter comprises a plurality ofT-flip-flops.
 17. The CMOS image sensor of claim 16, wherein thecontroller further generates an inversion signal applied to the counterto generate the inverted first count value.
 18. The CMOS image sensor ofclaim 17, wherein a first T-flip-flop in the plurality of T-flip-flopscomprises: a clock terminal that receives the clock signal; a resetterminal that receives the reset signal; an inverting terminal thatreceives the inversion signal; and an output terminal that provides afirst output, and a second T-flip-flop connected behind the firstT-flip-flop in the plurality of T-flip-flops comprises: a clock terminalthat receives the first output; a reset terminal that receives the resetsignal; an inverting terminal that receives the inversion signal; and anoutput terminal that provides a second output.
 19. The CMOS image sensorof claim 18, wherein an output of the first T-flip-flop has a logicallyhigh level when the reset signal is applied to reset the pixel.
 20. TheCMOS image sensor of claim 18, wherein, when the inversion signal isactivated, the first T-flip-flop inverts data apparent at the outputterminal of the first T-flip-flop.
 21. A digital camera comprising: aprocessor; and a complementary metal oxide semiconductor (CMOS) sensorconnected to the processor via a bus, wherein the CMOS sensor comprises:a controller that generates a clock signal and a reset control signal; apixel that outputs a first pixel signal in response to the reset controlsignal resetting the pixel, and outputs a second pixel signal inresponse to external image data applied to the pixel; and an analog todigital converter comprising: a comparator that compares a first signalrelated to the first pixel signal to a ramp signal and first activates acomparison signal in response to the comparison between the first signaland the ramp signal, and that compares a second signal related to thesecond pixel signal to the ramp signal and again activates thecomparison signal in response to the comparison between the secondsignal and the ramp signal; and a counter that counts in response to theclock signal until the comparison signal is first activated to generatea first count value, then inverts the first count value to generate aninverted first count value, and then again counts in response to theclock signal until the comparison signal is again activated to generatea second count value.
 22. The digital camera of claim 21 , wherein thecounter is a single counter circuit configured to count in one directionfrom a reset value of the counter in response to the clock signal andgenerate the first count value, and count in the one direction from theinverted first count value in response to the clock signal and generatethe second count value.
 23. The digital camera of claim 21, wherein theCMOS imager sensor further comprises a buffer that receives and storesthe second count value and provides the second count value to thecontroller.
 24. The digital camera of claim 23, further comprising: amemory that receives the second count value from the controller via thebus under the control of the processor; and a display that displays animage corresponding in part to the second count value as received underthe control of the processor from the memory via the bus.